U.S. patent number 8,068,905 [Application Number 11/205,373] was granted by the patent office on 2011-11-29 for method and apparatus for continuous electrode impedance monitoring.
This patent grant is currently assigned to Compumedics Limited. Invention is credited to Warwick Freeman, Philip Grasso, Richard Newman.
United States Patent |
8,068,905 |
Freeman , et al. |
November 29, 2011 |
Method and apparatus for continuous electrode impedance
monitoring
Abstract
In one embodiment, the present invention includes a test signal
generator capable of producing an impedance test signal comprising
of a sine wave having a known frequency. The test signal generator
may include a crystal oscillator, a counter, and a lookup table.
The lookup table output is applied to a digital to analog converter
and is then low pass filtered using a conventional analog filter to
produce a sine wave of a known frequency and voltage amplitude. The
test signal flows through the electrode and combines with an
electrophysiological signal to form a combined signal. A signal
processor is used to isolate the combined signal into the test
signal component and the electrophysiological component. The signal
processor digitally low pass filters the combined signal and the
output of the low pass filter is the electrophysiological signal.
The signal processor then digitally bandpass filters the combined
signal using a filter with a center frequency which is the same as
the test frequency. The output of this filter is then used to
calculate the electrode impedance.
Inventors: |
Freeman; Warwick (Box Hill,
AU), Grasso; Philip (Box Hill, AU), Newman;
Richard (Templestowe, AU) |
Assignee: |
Compumedics Limited
(Abbotsford, Victoria, AU)
|
Family
ID: |
35658228 |
Appl.
No.: |
11/205,373 |
Filed: |
August 17, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060020218 A1 |
Jan 26, 2006 |
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Current U.S.
Class: |
600/547; 600/546;
324/602; 600/544; 600/509 |
Current CPC
Class: |
A61B
5/24 (20210101); A61B 5/276 (20210101); A61B
5/6843 (20130101) |
Current International
Class: |
A61B
5/05 (20060101); A61B 5/04 (20060101); G01R
27/02 (20060101) |
Field of
Search: |
;600/544-548,509-525
;324/600-727 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hindenburg; Max
Assistant Examiner: Pani; John
Attorney, Agent or Firm: Briggs and Morgan, P.A.
Claims
We claim:
1. A method comprising: acquiring a physiological signal through an
electrode sensor in contact with a location on a patient's body;
providing a frequency stable signal; clocking an input of a counter
with the frequency stable signal to produce a counter output,
accessing a lookup table with the counter output to produce a
lookup table output; applying the lookup table output to a digital
to analog converter to produce a digital to analog converter
output; low pass filtering the digital to analog converter output
to produce an impedance test signal comprising a sine wave having a
voltage amplitude, V.sub.z, and a known frequency which is higher
than a frequency range of the physiological signal being monitored;
converting the voltage amplitude, V.sub.z, into a test current,
I.sub.e1; at the same time as said acquiring, sending the test
current, I.sub.e1, from the electrode sensor in contact with the
location on the patient's body to produce a test signal having a
voltage component, V.sub.e1; receiving a combined signal comprising
the physiological signal and the test signal, wherein the
physiological signal has a voltage component, V.sub.s1; low pass
filtering the combined signal to isolate the voltage component,
V.sub.s1, of the physiological signal; band pass filtering the
combined signal to isolate the voltage component, V.sub.e1, of the
test signal and create a band pass filter output; calculating a
contact impedance between the electrode sensor and the patient by
dividing the band pass filter output by the test current, L.sub.e1;
and simultaneously displaying the physiological signal and the
contact impedance.
2. The method of claim 1, wherein the physiological signal is
selected from the group consisting of an electroencephalogram (EEG)
signal, an electrocardiogram (ECG) signal, an electrooculogram
(EOG) signal, and an electromyogram (EMG) signal.
3. The method of claim 1, wherein converting the voltage amplitude,
V.sub.z, into the test current, I.sub.e1, includes using a resistor
with a resistance that is higher than a desired electrode impedance
range.
4. The method of claim 1, further comprising activating an
indicator if the contact impedance exceeds a pre-determined
threshold.
5. A method comprising: acquiring a physiological signal through an
electrode in contact with a location on a patient's body, said
physiological signal being communicated via a line to a monitor
system, wherein the physiological signal has a voltage component,
V.sub.s1; providing a frequency stable signal; clocking an input of
a counter with the frequency stable signal to produce a counter
output; accessing a lookup table with the counter output to produce
a lookup table output; applying the lookup table output to a
digital to analog converter to produce a digital to analog
converter output; low pass filtering the digital to analog
converter output to produce an impedance test signal comprising a
sine wave having a voltage amplitude, V.sub.z, and a known
frequency which is higher than a frequency range of the
physiological signal; converting the voltage amplitude, V.sub.z,
into a test current, I.sub.e1; during said acquiring, sending the
test current, I.sub.e1, from the electrode in contact with the
location on the patient's body to produce a test signal having a
voltage component, V.sub.e1, said test signal being communicated
via said line; combining the physiological signal with the test
signal to create a combined signal; at said monitor system,
deriving a physiological waveform from the combined signal by low
pass filtering the combined signal to isolate the voltage
component, V.sub.s1, of the physiological signal; band pass
filtering the combined signal to isolate the voltage component,
V.sub.e1, of the test signal and create a band pass filter output;
calculating a contact impedance between the electrode and the
patient by dividing the band pass filter output by the test
current, I.sub.e1; and simultaneously displaying the physiological
waveform and the contact impedance from the combined signal.
6. The method of claim 5, wherein the physiological signal is
selected from the group consisting of an electroencephalogram (EEG)
signal, an electrocardiogram (ECG) signal, an electrooculogram
(EOG) signal, and an electromyogram (EMG) signal.
7. The method of claim 5, wherein converting the voltage amplitude,
V.sub.z, into the test current, I.sub.e1, includes using a resistor
with a resistance that is higher than a desired electrode impedance
range.
8. The method of claim 5, further comprising activating an
indicator if the contact impedance exceeds a pre-determined
threshold.
Description
FIELD OF THE INVENTION
Generally, the present invention relates to a method and apparatus
for ensuring the accuracy of an acquired physiological signal. More
specifically, the present invention is a method of monitoring
electrode impedance while receiving an electromagnetic
physiological signal.
BACKGROUND OF THE INVENTION
Physiological monitors will often have a testing procedure to check
whether the interface between a sensor and the patient being
monitored is adequate to acquire a physiological reading. This is
especially true with regards to the acquisition of an
electrophysiological signal. Typically, an electrophysiological
signal is acquired through an electrode which is attached to the
patient. The contact between an electrode and a patient's skin can
significantly affect the results of an electrophysiological signal.
High contact impedance generally causes poor quality recordings due
to power interference.
It is a common practice to measure the electrode to skin contact
impedance before the start of an electrophysiological recording
session. This is done by injecting a small alternating current, Ie,
into the electrode and measuring the voltage, Ve, produced across
the electrode. The electrode impedance, Ze, may be calculated from
the equation Ze=Ve/Ie.
However, most physiological monitors cannot monitor physiological
signals during an impedance test because the contact impedance test
interferes with the acquisition of the electrophysiological signal.
Electrophysiological signals such as EEG, ECG, EOG and EMG are
often distorted by the test current utilized during the test.
Consequently, the prior art devices have been unable to
continuously monitor the contact impedance between the electrode
and the patient.
It is not uncommon for electrodes to partially or fully detach from
a patient during monitoring, and such an occurrence can seriously
distort the electrophysiological signal acquired from the patient.
Furthermore, it is often too difficult to visually monitor each
electrode on a patient. Consequently, there is a need for an
apparatus and a method to continuously monitor the contact
impedance between an electrode and a patient without preventing the
acquisition of an electrophysiological signal.
SUMMARY OF THE INVENTION
The present invention is a method and apparatus for continuously
monitoring a test signal while simultaneously acquiring a
physiological signal. The impedance test methods currently employed
in electrophysiological recording equipment cause interference to
the signal being recorded because the test signal has a frequency
(or frequencies in the case of non-sinusoidal test waveforms)
within the frequency band of the electrophysiological signal. By
shifting the test signal to a slightly higher frequency than the
electrophysiological signal a digital signal processor (DSP) can be
used to low pass filter a received signal to recover and discern
the electrophysiological signal from the test signal. A digital
band pass filter of the DSP can be used to extract the impedance
test signal and electrode impedance from the received signal.
In one embodiment, the present invention includes a test signal
generator capable of producing an impedance test signal comprising
of a sine wave having a known frequency. The test signal generator
includes a crystal oscillator, a counter, and a lookup table. The
lookup table output is applied to a digital to analog converter and
is then low pass filtered using a conventional analog filter to
produce a test signal comprised of a sine wave having a known
frequency and voltage amplitude. The test signal is passed through
the electrode and combines with an electrophysiological signal to
form a combined signal.
In one embodiment, a signal processor is used to isolate the
combined signal into the test signal component and the
electrophysiological component. The signal processor digitally low
pass filters the combined signal and the output of the low pass
filter is the electrophysiological signal. The signal processor
then digitally bandpass filters the combined signal using a filter
with a center frequency which is the same as the test frequency.
The output of this filter is then used to calculate the electrode
impedance.
In one embodiment, the present invention can be adapted to be
integrated into an electrophysological monitoring system such as
EEG, EOG, EMG, and ECG. The contact impedance between an electrode
and a patient can be continuously monitored while simultaneously
acquiring an electrophysiological signal. A display can be used to
monitor both the physiological signal and the contact
impedance.
BRIEF DESCRIPTION OF THE DRAWINGS
In this disclosure, only certain embodiments are depicted, and not
all or every insubstantial change or modification of each such
embodiment are depicted or described herein, although those of
ordinary skill in the art to which the invention is directed will
appreciate many insubstantial changes to the teaching of this
disclosure that nevertheless fall within the spirit and scope of
the invention. In this disclosure, reference numerals are used to
refer to various elements, sub-elements and process steps and the
same reference numeral is intended to denote all similar or
identical elements set forth herein.
FIG. 1 is a block diagram of one embodiment of the present
invention.
FIG. 2 is a graph of the relationship between a test frequency and
a frequency of an electrophysiological signal.
FIG. 3 is a block diagram of a system employing one embodiment of
the present invention.
FIG. 4 is an embodiment of a display in one embodiment of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a method and apparatus for change in
electrode impedance monitoring via a combined test and
physiological signal. While the embodiment disclosed is
particularly adapted to monitor contact impedance and EEG, ECG,
EOG, or EMG signals, one skilled in the art can readily adapt the
present invention to monitor different parameters, which involve
the testing of a sensor that is acquiring a physiological
signal.
As shown in FIG. 1, in one embodiment, the present invention
includes a test signal generator capable of producing an impedance
test signal comprising of a sine wave having a known frequency, fz
3, which is slightly higher than the frequency range of the
electrophysiological signal being monitored. A crystal oscillator
12 provides a known, frequency stable signal to clock the input of
a counter 14. The counter output sequentially accesses a lookup
table 16 which can be implemented using any digital storage device
such as an EPROM or RAM, containing a sine waveform in digital
format. The lookup table 16 output is then applied to a digital to
analog converter (DAC) 18. The output of the DAC is low pass
filtered using a conventional analog filter 20 to produce a sine
wave of frequency, fz, and voltage amplitude Vz.
A resistor, Rd1, with a resistance many times higher than the
desired electrode impedance range, converts Vz into a test current
Ie1. This current flows through the electrode, represented in FIG.
1 by Ze1, to produce a voltage, Ve1, at the input of amplifier 22.
There will also be an electrophysiological signal, Vs1, at the
input of amplifier 22. The combined signal of Ve1+Vs1 is amplified
and then low pass filtered by anti-aliasing filter 24 before being
converted into a digital signal by analog to digital converter
(ADC) 26. The resultant digital signal is read by a digital signal
processor (DSP) 30 via multiplexor 28. FIG. 1 shows the concept of
the present invention extended to n electrode channels, using a
separate ADC 26 for each channel, but a single ADC with an analog
multiplexor would work equally well.
The ADC sampling frequency should be greater than twice fz to
prevent aliasing. The DSP 30 should have sufficient computational
power to execute both filters for all channels at the desired
sample rate plus any storage or display functions. It should be
noted that the DSP 30 could alter the sampling frequency, fs,
signal bandwidth, fo, and impedance test frequency, fz, provided
the relationship to each other is maintained as per FIG. 2.
In one embodiment, the combined signal can be isolated into the
test signal component and the electrophysiological component by
filtering the combined signal at appropriate frequencies. The DSP
30 digitally low pass filters the combined signal using a filter
with a -3 dB point, fo, which is lower than the impedance test
frequency, fz as shown in FIG. 2. The physiological signal, Vs1,
alone is the output of the low pass filter. The low pass filter
should have a sharp roll-off characteristic so that the test signal
component at fz is completely removed. The filter should also have
a linear phase characteristic so the physiological signal is not
distorted. A symmetrical FIR filter (finite impulse response) can
be readily designed to meet both these requirements.
The DSP 30 also digitally bandpass filters the combined signal
using a filter with a center frequency of fz, the same as the test
frequency. The output of this filter is Ve1, as the physiological
signal and any higher frequency noise has been removed by the
bandpass filter. The bandpass filter may be implemented as either
an FIR or an IIR (infinite impulse response) if shorter computation
time is needed.
The electrode impedance, Ze, can then be calculated as Ze=Ve1/Ie1,
where Ie1=Vz/Rd1, which is a constant. In one example Vz would be 1
volt pk-pk and Rd1=20 megohms, giving Ie1=50 nanoampers pk-pk. Thus
Ve1 will be 50 microvolts pk-pk per kilohm of electrode impedance.
The impedance of each electrode could be displayed numerically on a
computer monitor connected to the DSP (FIGS. 3 and 4) or used to
activate indicators such as light emitting diodes attached to the
amplifier circuit enclosure should the impedance exceed a
pre-determined threshold.
As shown in FIGS. 3 and 4, in one embodiment, the present invention
can be adapted to be integrated into an electrophysiological
monitoring system 30 such as EEG, EOG, EMG, and ECG. The contact
impedance between an electrode and a patient can be continuously
monitored while simultaneously acquiring an electrophysiological
signal. A display 32 can be used to monitor both the physiological
signal and the contact impedance.
In one embodiment, the electrophysiological monitor system 30 may
also communicate with a central monitoring station 34. The
electrophysiological monitor system 30 is adapted to trigger an
alarm condition at the central monitoring station 34 should the
impedance of any of the electrodes exceed a pre-determined
threshold for a pre-determined time (to avoid spurious values
triggering the alarm). The operator could set the alarm threshold
and the minimum time the threshold needs to be exceeded before the
alarm is triggered via a computer network connection to the
electrophysiological monitor system 30. This would allow an
operator to monitor the contact impedance of each electrode for
each patient at a central location remote from the
electrophysiological monitor system 30.
The matter set forth in the foregoing description and accompanying
drawings is offered by way of illustration only and not as a
limitation. While a particular embodiment has been shown and
described, it will be obvious to those skilled in the art that
changes and modifications may be made without departing from the
broader aspects of applicant's contribution. The actual scope of
the protection sought is intended to be defined in the following
claims when viewed in their proper perspective based on the prior
art.
* * * * *